Project Summary The long-term goal of this research is to understand the mechanisms that regulate the assembly, targeting and activity of microtubule motors during intracellular transport. Intracellular transport of cellular cargo is a fundamental process underlying cell growth and differentiation. Malfunctions associated with mutations in the transport machinery give rise to many developmental and neurological diseases. To understand how the microtubule motors, cytoplasmic dynein and kinesins, find and transport cargoes along microtubule tracks to specific cellular destinations, we need to know more about how motors are attached to, and released from, cargoes, how tracks are specified and maintained, and how the activity of cargo motor ensembles is coordinated in space and time. In this grant period, we will capitalize on the well characterized motor neurons of Drosophila and the sophisticated genetic tool box available, to query the intact organism for insights into how these highly conserved motors and cellular transport mechanisms are coordinated. In Aim 1 we are analyzing Spinocerebellar Ataxia Type 5 (SCA5) mutations linked to neurological disease to understand how spectrin functions in intracellular transport. We have found that mutations in ?-spectrin impair synaptic vesicle and organelle motility in neurons and disrupt neuronal morphogenesis. We discovered that a SCA5 mutation in the actin-binding domain of ?-spectrin causes it to bind actin with a 1000-fold increased affinity. Subsequent structural studies predict a similarly elevated affinity between ?-spectrin and the Arp1 filament of dynactin. We will determine if the loss of dynamic binding between ?-spectrin on the surface of a vesicle and the Arp1 subunit of the dynactin motor adapter, underlies SCA5-induced synaptic vesicle motility defects. In Aims 2 and 3, we are identifying novel genes and pathways that regulate synaptic function using transport assays in motor neurons. We recently showed that a protein complex, Striatin-interacting phosphatase and kinase (STRIPAK), regulates axonal transport, and is in a complex with dynein. We will study how STRIPAK components mediate activation of axonal transport. In addition, we have uncovered evidence of trans- synaptic regulation of axonal transport involving the postsynaptic cytoskeleton. We will investigate the signaling mechanisms involved in the regulation of synaptic homeostasis by the postsynaptic cytoskeleton. Our studies will provide new insights in the mechanisms that regulate transport and potential avenues for the treatment of neurological diseases.